1. Field of the Invention
The present invention relates to diagnostic and therapeutic compositions, methods of their use, and processes of their preparation.
More particularly, the invention relates to:                (a) Magnetic resonance diagnostic compositions for visualization of tissues that over-express folate binding protein, comprising ligands chelated to superparamagnetic or paramagnetic metals and coupled to folate-receptor binding ligands;        (b) Radiodiagnostic compositions for visualization of tissues, comprising ligands chelated to radioactive gamma-emitting metals and coupled to folate-receptor binding ligands;        (c) Compositions for radiotherapy or for neutron capture therapy, comprising ligands chelated to radioactive alpha or beta-emitting metals or to metals suitable for neutron capture therapy and coupled to folate-receptor binding ligands; and        (d) Compositions for chemotherapy, comprising certain derivatives of folic acid coupled to a cancer chemotherapy drug through the alpha carboxylate of folic acid or coupled through both the alpha and gamma carboxylates.        
2. Reported Developments
The folate-based diagnostic and therapeutic agents of the present application are designed for use in Nuclear Medicine, Magnetic Resonance Imaging (MRI), and neutron capture therapy applications. Magnetic resonance (hereinafter sometimes referred to as MR) imaging is well known and widely used by the prior art for obtaining spatial images of parts of a patient for clinical diagnosis. The image is obtained by placing the patient in a strong external magnetic field and observing the effect of this field on the magnetic properties of protons contained in and surrounding the organ or tissue of the patient. The proton relaxation times, called T1 or spin-lattice or longitudinal relaxation time, and T2 or spin-spin or transverse relaxation time depend on the chemical and physical environment of the organ or tissue being imaged. In order to improve the clarity of the image, a diagnostic agent is administered intravenously (hereinafter sometimes referred to as I.V.) and is taken up by the organs, such as the liver, spleen, and lymph nodes to enhance the contrast between healthy and diseased tissues.
The contrast agents used in MR imaging derive their signal-enhancing effect from the inclusion of a material exhibiting paramagnetic, ferrimagnetic, ferromagnetic or superparamagnetic behavior. These materials affect the characteristic relaxation times of the imaging nuclei in the body regions into which they distribute causing an increase or decrease in MR signal intensity. There is a need for contrast agents such as those of the present invention, that selectively enhance signal intensity in particular tissue types, as most MR contrast agents are relatively non-specific in their distribution.
Nuclear medicine procedures and treatments are based on internally distributed radioactive materials, such as radiopharmaceuticals or radionuclides, which emit electromagnetic radiations as gamma rays or photons. Following I.V., oral or inhalation administration, the gamma rays are readily detected and quantified within the body using instrumentation such as scintillation and gamma cameras. The gamma-emitting agents of the present invention are designed to selectively localize in particular targeted tissues by transmembrane transport, yielding either high signal intensity in these tissue types for imaging purposes, or high radiation dose, for radiotherapy purposes.
Transmembrane transport of exogenous molecules, such as diagnostic agents, is also known by the prior art. One method of transmembrane delivery, receptor-mediated endocytosis, is the movement of extracellular ligands bound to cell surface receptors into the interior of the cells through invagination of the membrane. This process is initiated by the binding of a ligand to its specific receptor. Folates, which are required for the survival and growth of eukaryotic cells, are taken up into cells by receptor-mediated transport after binding to folate binding protein on the cell membrane. The cellular uptake of exogenous molecules can be enhanced by conjugation of these molecules to folate. Such conjugates have been used to target folate receptors to enhance cellular uptake of exogenous molecules, including some diagnostic agents. The uptake of substances by receptor-mediated endocytosis (hereinafter sometimes termed RME) is a characteristic ability of some normal, healthy cells. RME transport systems have been found on normal macrophages, hepatocytes, fibroblasts and reticulocytes. On the other hand, conversion of normal cells into tumor cells can be associated with an increase or decrease in the activity of receptors performing RME or, sometimes, with changes in the levels of receptor expression.
The use of neutron capture therapy for the treatment of cancer is well known to those skilled in the art. Briefly the system comprises administering a target substance that emits short-range radiation when it is irradiated with neutrons. Boron-10 has traditionally been used for neutron capture therapy, but more recently Gadolinium-157, which has a very high cross section for neutrons and emits short range Auger-electrons, has been used. [Brugger, R. M. and Shih, J. A., Strahlentherapie Und Onkologie, 165, 153-156, 1989; Brugger, R. M. and Shih, J. A., Medical Physics, 19, 733-744, 1992]. Specificity is achieved by using neutrons of appropriate energy and the selective distribution of the gadolinium within the tumor tissue. In the past, neutron capture therapy has suffered from insufficient concentration of target substance in the desired cells and in the case of gadolinium, has suffered from the exclusion of the gadolinium from the inside of the cell. The use of the folate-containing gadolinium compounds of this invention is advantageous because of the large amounts of gadolinium that are specifically taken up by the desired cells. The internalization of the compounds of this invention following binding to folate binding protein is beneficial because of the short range of the Auger electrons. In addition, the gadolinium compounds of this invention can be used as MRI contrast agents that selectively target the cells that are to be treated by neutron capture therapy. The imaging data can provide the radiotherapist with spatial information beneficial for planning the radiotherapy procedure, using the same gadolinium atoms as are used as the target for the neutrons.
The following illustrative studies describe relevant properties of the folate receptor.
Folic acid or pteroyl glutamic acid is a vitamin consisting of a pteridine ring linked by a methylene bridge to a para-aminobenzoic acid moiety, which is joined through an amide linkage to a glutamic acid residue. Folic acid and folates are well absorbed from the diet primarily via the proximal portion of the small intestine. Following their absorption from the digestive system, dietary folates are rapidly reduced by dihydrofolate reductase and other enzymes to tetrahydrofolic acid and derivatives thereof.
Folates are required for the survival and growth of eukaryotic cells, so their cellular uptake is assured by at least two independent transport mechanisms. Reduced folates are internalized via a carrier-mediated low affinity (Km 1-5 μM) anion-transport system that is found in nearly all cells. Folic acid and 5-methyl tetrahydrofolate can also enter cells via a high affinity (Kd values in the nanomolar range) membrane-bound folate-binding protein (hereinafter sometimes referred to as FBP) that is anchored to the cell membrane via a glycosylphosphatidylinositol (hereinafter sometimes referred to as GPI) moiety. This process has been studied in MA104 cells, where experiments have shown that 5-methyltetrahydrofolate is taken up into the cell after binding to glycosylphosphatidylinositol (GPI)-anchored FBP that has clustered in cell structures known as caveolae. The caveolae then seal the folate binding protein-folate complex off from the extracellular space and transport folate into the cell. Once inside, the folate dissociates from FBP and diffuses into the cytoplasm, where it is rapidly coupled to one or more glutamic acid residue, slowing diffusion out of the cell. The caveolae and FBP then migrate to the membrane surface for another round of folate uptake.
There are two major isoforms of the human membrane folate binding proteins, α and β. The two isoforms have ˜70% amino acid sequence homology, and differ dramatically in their stereospecificity for some folates. Both isoforms are expressed in both fetal and adult tissue; normal tissue generally expresses low to moderate amounts of FR-β. FR-α is expressed in normal epithelial cells and is frequently strikingly elevated in a variety of carcinomas, with the exception of squamous cell carcinomas of the head and neck. Several papers have reported the overexpression of folate binding protein in cancer. See for example:    Ross J F, Chaudhuri P K, Ratman M, “Differential regulation of folate receptor isoforms in normal and malignant tissues in vivo and in established cell lines. Physiologic and clinical implications”, Cancer, 1994, 73(9), 2432-2443;    Rettig, W, Garin-Chesa P, Beresford H, Oettgen H, Melamed M. Old L., “Cell-surface glycoproteins of human sarcomas: differential expression in normal and malignant tissues and cultured cells”, Proc. Natl. Acad. Sci U.S.A., 1988, 85, 3110-3114;    Campbell I G, Jones T A, Foulkes W D, Trowsdale J., “Folate-binding protein is a marker for ovarian cancer”, Cancer Res., 1991, 51, 5329-5338;    Coney L R, Tomassetti A, Carayannopoulos L, Frasca V, Kamen B A, Colnaghi M I, Zurawski V R Jr, “Cloning of a tumor-associated antigen: MOv18 and MOv19 antibodies recognize a folate-binding protein”, Cancer Res. 1991, 51, 6125-6132;    Weitman S D, Lark R H, Coney L R, Fort D W, Frasca V, Zurawski V R Jr, Kamen B A, “Distribution of the folate receptor (GP38) in normal and malignant cell lines and tissues”, Cancer Res., 1992, 52, 3396-3401;    Garin-Chesa P, Campbell I, Saigo P, Lewis J, Old L, Rettig W, “Trophoblast and ovarian cancer antigen LK26. Sensitivity and specificity in immunopathology and molecular identification as a folate-binding protein”, Am. J. Pathol., 1993, 142, 557-567;    Holm J, Hansen S I, Hoier-Madsen M, Sondergaard K, Bzorek M, “Folate receptor of human mammary adenocarcinoma”, APMIS, 1994, 102, 413-419;    Franklin W A, Waintrub M., Edwards D, Christensen K, Prendergrast P, Woods J., Bunn P A, Kolhouse J F, “New anti-lung cancer antibody cluster 12 reacts with human folate receptors present on adenocarcinoma”, Int. J. Cancer, 1994, 8 (Suppl.) 89-95.    Miotti S, Canevari S, Menard S, Mezzanzanica D, Porro G, Pupa S M, Regazzoni M, Tagliabue E, and Colnaghi M I, “Characterization of human ovarian carcinoma-associated antigens defined by novel monoclonal antibodies with tumor-restricted specificity”, Int. J. Cancer, 1987, 39, 297-303; and    Vegglan R, Fasolato S, Menard S, Minucci D, Pizzetti P, Regazzoni M, Tagliabue E, Colnaghi M I, “Immunohistochemical reactivity of a monoclonal antibody prepared against human ovarian carcinoma on normal and pathological female genital tissues”, Tumori, 1989, 75, 510-513.
Folate binding proteins are also present in normal adult oviduct epithelium and in kidney distal and proximal tubules, where they serve to prevent excessive loss of folate via the urine. Kidneys may, as a result, be a significant source of toxicity. Folic acid in high doses has been reported to be nephrotoxic and a kidney-specific tumor promoter, as it is rapidly concentrated in the kidney and precipitated in the tubules as urinary pH drops, causing obstructive nephropathy. This injury results in diffuse renal cell proliferation and hypertrophy. Rats given i.v. injections of folic acid (250 mg/kg) in 0.3 M sodium bicarbonate showed an increase in the ratio of kidney to body weight that reached 165% of control by 24 h after treatment. See for example:    Klinger E L J, Evan A P, Anderson R E, “Folic acid-induced renal injury and repair”, Arch. Pathol. Lab. Med. 1980, 104, 87-93;    Hsueh W. Rostorfer H H, “Chemically induced renal hypertrophy in the rat”, Lab. Invest. 1973, 29, 547-555; and    Dong L. Stevens J L, Fabbro D, Jaken S, “Regulation of Protein Kinase C isozymes in kidney regeneration”, Cancer Res. 1993, 53, 4542-4549.
Overexpression of FBP by a number of different tumors has led a number of investigators to explore its potential as a delivery system for toxins or poorly permeable compounds coupled to folic acid and as a means to increase selective delivery of antifolate drugs such as methotrexate to tumors. The amount of FBP on the membrane of ovarian cancer cells is high (1×106 molecules/cell). IGROV cells in culture can bind 3H folic acid at a level of 10-12 pmol/106 cells; MA104 cells bind 1-2 pmol folic acid/106 cells. FBP has a very high affinity for folic acid and some of its reduced folate cofactors (Kd˜1-10 nM); this presumably favors folate uptake at the usual folate concentrations that exist in vivo (5-50 nM). The recycling rate for the folate binding protein (in vitro) has been reported to range from ˜30 min in MA104 cells to 5 hr in L1210 cells. Several antifolate drugs have been shown to bind to FBP; these compounds, of which methotrexate is characteristic, have been used to antagonize the growth of cancer cells. See, for example:    Orr R B, Kamen B A, “UMSCC38 cells amplified at 11q13 for the folate receptor synthesize a mutant nonfunctional folate receptor”, Cancer Res. 1994, 54, 3905-3911;    Anthony A C, “The biological chemistry of folate receptors”, Blood, 1992, 79, 2807-2820; and    Spinella M J, Brigle K E, Sierra E E, Goldman, I D, “Distinguishing between folate receptor-α-mediated transport and reduced folate carrier-mediated transport in L1210 leukemia cells”, J. Biol. Chem., 1995, 270, 7842-7849.
These studies indicate an essential fact necessary to distinguish between normal cells and tumor cells when delivering pharmaceutical or diagnostic agents into a patient using folates to be internalized by FBP. FBP levels are low in many normal tissue types while, in comparison, FBP levels are high in many tumor cells. This difference between the folate receptor levels allows selective concentration of pharmaceutical or diagnostic agents in tumor cells relative to normal cells, thereby facilitating treatment or visualization of tumor cells.
In culture, cells were successfully targeted through FBP using folate-conjugated protein toxins that would not normally penetrate the cell membrane through diffusion, as well as with folate-derivatized drug/antisense oligonucleotide-carrying liposomes. See, for example:    Leamon C P, Low P S, “Cytotoxicity of momordin-folate conjugates in cultured human cells”, J. Biol. Chem., 1992, 267, 24966-24967;    Leamon C P, Paston I, Low P S, “Cytotoxicity of folate-pseudomonas exotoxin conjugates towards tumor cells”, J. Biol. Chem., 1993, 268, 3198-3204;    Lee R J, Low P S, “Delivery of liposomes into cultured KB cells via folate receptor-mediated endocytosis”, J. Biol. Chem., 1994, 269, 3198-3204;    Wang S, Lee R J, Cauchon G, Gorenstein D G, Low P S, “Delivery of antisense oligonucleotides against the human epidermal growth factor receptor into cultured KB cells with liposomes conjugated to folate via polyethyleneglycol”, Proc. Natl. Acad. Sci U.S.A., 1995, 92, 3318-3322; and    Wang S, Lee R J, Mathias C J, Green M A, Low P S, “Synthesis, purification and tumor cell uptake of Ga-67-Deferoxamine-folate, a potential radiopharmaceutical for tumor imaging”, Bioconj. Chem., 1996, 7, 56-63.
The prior art has spent considerable energy in studying folate binding protein as a potential target for delivery of exogenous molecules into cells that express folate binding protein, as further illustrated hereunder.
U.S. Pat. No. 5,416,016 and WO 96/36367 (Low et al.) are directed to a method for enhancing transmembrane transport of exogenous molecules and disclose such delivery wherein the method comprises: contacting a membrane of a living cell with a complex formed between an exogenous molecule and a ligand of folic acid and folate analogs to initiate receptor-mediated transmembrane transport of the ligand complex. The exogenous molecules include a large variety of compounds, peptides, proteins and nucleic acids, analgesics, antihypertensive agents, antiviral agents, antihistamines, cancer drugs, expectorants, vitamins, plasmids and diagnostic agents.
The synthetic methods described in these documents were not regioselective, and mixtures containing folic acid coupled to the exogenous molecule through either the α- or γ-carboxylate of folate are expected to form. In the process disclosed in U.S. Pat. No. 5,416,016 these mixtures were not separated.
WO 96/36367 distinguishes between the two isomers of DF-folates, i.e., those where deferoxamine is coupled to the folate moiety through the α- or through the γ-carboxyl group of folate, based on their competition with free folate for the cell surface FBP: it was found that the ax-conjugate was unable to compete with free folate for the cell surface FBP. In a comparative test a 50% decrease in bound [3H] folic acid was observed in the presence of an equimolar amount of the DF-folate (γ) conjugate, while the DF-folate (α) isomer displayed no ability to compete with the radiolabeled vitamin.
Wang et al., supra, studied the uptake of 67Ga-deferoxamine-folate into KB tumor cells (a human nasopharyngeal epidermal carcinoma cell line that greatly overexpresses the folate binding protein) as a potential radiopharmaceutical. When 0.15 μCi (100 pmol) of 67Ga-DF-folate (deferoxamine coupled to folic acid via the γ-carboxylate of folate) was incubated with monolayers of KB cells, the final % uptake of the compound by the KB cells was 32% of the applied radioactivity. The compound had very low non-specific binding as indicated by very low activity levels bound to a receptor-negative cell line control.
Wang et al. subsequently published another report* stating that folic acid derivatives that are modified at the alpha carboxylate have no affinity for cell surface folate receptors. They reported the preparation of FITC-EDA-folate derivatives containing a fluorescein moiety (FITC) linked to folate through either the α- or γ-carboxylate of folate (via an ethylenediamine [EDA] spacer). The two isomers were incubated with KB cells that overexpress FBP. The cells were then washed to remove unbound compound and assayed for cell-associated fluorescence. The γ-isomer of FITC-EDA-folate showed half maximal binding to KB cells at a concentration of 1.6 nM (binding comparable to native folate), but the α-isomer of FITC-EDA-folate had “virtually no affinity for the cell surface receptors”.    *Wang, Susan; Luo, Jin; Lantrip, Douglas A.; Waters, David J.; Mathias, Carla J.; Green, Mark A.; Fuchs, Philip L.; Low, Philip S. Design and Synthesis of [111In]DTPA-Folate for Use as a Tumor-Targeted Radiopharmaceutical. Bioconjugate Chem. (1997), 8(5), 673-679.
The folate-based agents of the present application were designed for use in nuclear medicine, neutron capture therapy, or MRI applications. Based on the teachings in WO96/36367 that only folate adducts coupled to exogenous molecules through the gamma carboxylate of folate are recognized by FBP, we devised regiospecific syntheses for the preparation of these folate conjugates, rather than using the non-regiospecific methods used by others. The conjugates prepared by these regiospecific routes contained metal chelating ligands coupled to folate through its gamma carboxylate. The corresponding alpha isomers were prepared for use as negative controls. Surprisingly, when the ability of the alpha and gamma isomers to bind to FBP in tumor cells in vitro was compared, the alpha isomers (our “negative” controls) bound to FBP to the same extent as the gamma isomers in a variety of in vitro studies (vide infra). This result was surprising in light of the reports of Wang et al. Also surprising was our subsequent finding that folate compounds derivatized with metal chelates at both the alpha and gamma carboxylate of folate (bis derivatives) were also able to bind to FBP.
We also performed studies with the alpha and gamma isomers in tumor-bearing animals, where ability of the alpha isomers to localize in the tumors was surprisingly found to be equal to or greater than that observed with the corresponding gamma conjugates. In addition, the clearance behavior of the two isomers was compared, both in vivo and in vitro. As discussed in greater detail later, the urinary clearance of the alpha isomers from the body was significantly and unexpectedly higher than that observed with the corresponding γ-isomer or with 3H folate. This may be an advantage for some nuclear medicine and radiotherapy applications for these compounds, because retention in non-target organs causes higher radiation dose to the patient and lower target to background ratios. Compounds that are more rapidly excreted from the body provide an improved margin of safety.
We have also discovered that the alpha isomers of the folate conjugates of the present invention also show unexpectedly faster clearance from cells in vitro. Studies were performed to compare the clearance of metal complexes coupled to the γ- or α-carboxylate of folates or to both the α- and γ-carboxylates of folates (hereinafter sometimes termed bis derivatives) from KB and JAR cells. We obtained the surprising finding that the clearance rate of the α isomer and of the bis isomer from KB and JAR cells is significantly faster than that of the corresponding γ isomer or of 3H folate.
Based on this surprising discovery we have also found that the clearance rate of folate-based diagnostic agents designed for use in nuclear medicine or MRI applications can be varied or tailor-made by using various proportions of the α-isomer, the bis isomer and γ-isomer of such diagnostic agents. In addition to tailor-making the rate of clearance from certain organs, such as the kidney, liver, brain, liver, kidneys and from various tissues such as tumors that over-express folate binding protein, the use of chelating agents chosen for the compounds of the present invention provides a greater margin of safety against the toxicity of the metal used in the chelates.
Experiments from our laboratories on the cellular uptake of monomeric folate conjugates of Gd chelates designed for use in MR applications indicate that structural modifications that bring about an increase the intensity of the MR signal are advantageous, as the signal intensity obtainable with this technique is determined by the quantity of paramagnetic or superparamagnetic metal that can be localized in the target tissues. This is, in turn, limited by the quantity of folate binding protein present in those tissues. The desired increase in signal intensity could be achieved by attaching multimeric Gd chelates to a single folate residue and/or by the use of enhanced relaxivity Gd chelates, that are, as a result of their structure, expected to provide higher intrinsic signal intensity per Gd atom. Based on these observations the following concepts are presented for the design of new monomeric and multimeric folate conjugates of Gd chelates in order to enable MR imaging of tumors and other tissues that over-express the folate binding protein.